Implantable medical devices (IMDs) sense physiological signals for diagnosing a patient condition and/or managing the delivery of a medical therapy. One example of an IMD used for monitoring a patient is an implantable loop recorder (ILR) that records a patient's ECG subcutaneously for diagnosing pathologic conditions like fainting or transient arrhythmias. A pair of sense electrodes spaced apart on the device housing are used to sense the subcutaneous ECG. The ILR records the ECG signal when the patient, feeling symptomatic, activates the recording function of the ILR by holding a telemetry-enabled activator over the ILR and pressing a button. Alternatively, the detection of an arrhythmia by the ILR using arrhythmia detection algorithms may automatically trigger ECG signal storage.
Another IMD that relies on subcutaneously sensed ECG signals is a subcutaneous implantable cardioverter defibrillator (SubQ ICD). The SubQ ICD detects arrhythmias using the subcutaneously sensed ECG and delivers cardioversion/defibrillation (CV/DF) therapy in response to detecting malignant tachcyardias.
Subcutaneous ECG sensing in such devices is complicated by the presence of non-cardiac myopotentials (e.g., skeletal muscle activations in the vicinity of the implanted device), motion artifact, other physiological signals (i.e., respiration and gastrointestinal signals), electromagnetic interference (typically 50 or 60 Hz electrical noise from power mains), and electrostatic discharge (ESD). ESD is characterized by a large common mode or differential mode transient that saturates the input amplifier or overflows the input to an analog-to-digital converter, preventing the detection of cardiac signals for a period of several seconds. Noise in the subcutaneous ECG signal can interfere with the appropriate detection and response to cardiac arrhythmias or other cardiac conditions by implantable devices such as an ILR or SubQ ICD. The presence of noise in the ECG signal sensed by an ILR may trigger data storage inappropriately, causing the memory to filled with data of little use in diagnosing the patient's condition. Over sensing of noise in the subcutaneous ECG signal by a SubQ ICD may cause inappropriate tachycardia detection and unnecessary delivery of a CV/DF high voltage shock.